The purification of ribosomal RNA gene chromatin from Physarum polycephalum

Chromatin states at ribosomal DNA loci

Eukaryotic transcription of ribosomal RNAs (rRNAs) by RNA polymerase I can account for more than half of the total cellular transcripts depending on organism and growth condition. To support this level of expression, eukaryotic rRNA genes are present in multiple copies. Interestingly, these genes co-exist in different chromatin states that may differ significantly in their nucleosome content and generally correlate well with transcriptional activity. Here we review how these chromatin states have been discovered and characterized focusing particularly on their structural protein components. The establishment and maintenance of rRNA gene chromatin states and their impact on rRNA synthesis are discussed. This article is part of a Special Issue entitled: Transcription by Odd Pols.

Binding of histones to Xenopus laevis ribosomal genes with different levels of expression

The association of ribosomal RNA genes with histones as a function of their expression has been studied in Xenopus laevis erythrocytes, where the genes are silent, and tadpoles at stage 40, where these genes are actively transcribed. Isolated nuclei were either treated with formaldehyde or irradiated with an ultraviolet laser to cross-link proteins to DNA. The covalently linked protein-DNA complexes were purified by centrifugation through CsCl and immunoprecipitated with antibodies against H1, H2A and H4. DNA from the precipitated complexes was analysed for the presence of ribosomal DNA sequences by hybridization to specific probes. The actively transcribed ribosomal genes from X. laevis embryos are associated with H1, H2A and H4 as are the non-transcribed genes in the erythrocytes.